1. Field of the Invention
This invention relates to a signal processing device of a touch panel, specifically to a signal processing device of a touch panel using an electrostatic capacity.
2. Description of the Related Art
An electrostatic capacity type input device called a touch sensor or the like has been known as an input device to various kinds of electronic devices such as mobile phones, portable audio devices, portable game consoles, televisions and personal computers. This kind of touch sensor is disclosed in Japanese Patent Application Publication No. 2005-190950, for example.
A conventional touch sensor (signal processing device of a touch panel) is described referring to FIGS. 9 and 10. A touch pad 51 is formed on a PCB (Printed Circuit Board) substrate 50 so that an electrostatic capacitor 52 (capacitance C) is formed between the touch pad 51 and the PCB substrate 50, as shown in FIG. 9. The touch pad 51 is connected to a non-inverting input terminal (+) of a comparator 53 through a wiring 54. A reference voltage Vref is applied to an inverting input terminal (−) of the comparator 53. A constant current power supply 55 is connected to the wiring 54 that connects the touch pad 51 and the non-inverting input terminal (+) of the comparator 53.
Operations of the touch sensor are described referring to FIG. 10. When a finger 56 of an operator is far away from the touch pad 51, a capacitance attached to the touch pad 51 is C. In this case, a voltage at the touch pad 51 increases from 0 V in a reset state as the electrostatic capacitor 52 is charged with a constant current from the constant current power supply 55. An output voltage of the comparator 53 is inverted when the voltage at the touch pad 51 reaches the reference voltage Vref. A length of time from the reset state to the inversion of the comparator 53 in this case is referred to as t1.
When the finger 56 of the operator approaches the touch pad 51, on the other hand, the capacitance associated with the touch pad 51 increases to C+C′. The increment C′ is a capacitance of a capacitor formed between the finger 56 of the operator and the touch pad 51. As a result, the length of time that the voltage at the touch pad 51 takes from 0 V to the reference voltage Vref increases to t2 (t2>t1). Which means it is possible to detect whether the finger 56 of the operator has touched or not based on a difference (t2−t1) in the length of time required for the transition from the reset state to the inversion of the comparator 53. In other words, the touch pad 51 functions as an ON/OFF switch for data input.
In the conventional touch sensor, however, there is a problem that the voltage at the touch pad 51 is varied to cause malfunctioning of the touch sensor when a noise is applied to the touch pad 51.
In addition, an amount of data that can be inputted is limited because the touch pad 51 serves as a binary switch having an ON state and an OFF state.